We propose to use Drosophila as a model system to study the mechanisms involved in the control of retrotransposon mobilization. We will exploit a hybrid dysgenesis system in which unidirectional crosses between two specific strains of D. virilis result in the germline movement of different transposons. One of them is the Penelope element which, based on several lines of circumstantial evidence, seems to be the causative agent of this phenomenon. Penelope is a 2.7 kb element flanked by 690 bp inverted repeats and containing one open reading frame capable of encoding a protein homologous to the reverse transcriptase of retroviruses. A second characterized element mobilized in these crosses is the LTR-containing Ulysses retrotransposon. The first step in our analysis will be to demonstrate that Penelope is able to mobilize other transposable elements after germline transformation into D. melanogaster. Experiments will be designed to determine if a single copy of Penelope is sufficient to promote its mobilization or multiple copies of Penelope, or a single copy under the control of a heat shock promoter, are necessary to obtain this effect. We will then analyze whether Penelope encodes an enzymatically active reverse transcriptase and whether this element can move by a reverse transcription mechanism. The molecular basis for Penelope mobilization in the progeny of dysgenic crosses will be analyzed by studying the germline distribution of RNA(s) and protein(s) encoded by this element during the development of dysgenic flies. The second part of the proposal will concentrate on the analysis of the mechanisms of mobilization of the Ulysses LTR-containing retrotransposon. The expression of this element in the germline of dysgenic flies will be analyze to evaluate the possibility that high frequency of mobilization is due to increased transcription/translation of Ulysses-encoded products. We will then test the hypothesis that Ulysses mobilization is due to the use of Penelope-encoded reverse transcriptase by Ulysses VLPs. Alternative possibilities will be tested using an in vitro system to measure the ability of VLPs present in the germline of dysgenic flies to direct in vitro integration of Ulysses DNA.